Montana Tech Library Digital Commons @ Montana Tech Graduate Theses & Non-Theses Student Scholarship Summer 2019 MINERALOGY AND FLUID INCLUSION STUDY OF THE CRYSTAL MOUNTAIN FLUORITE MINE, RAVALLI COUNTY, MONTANA Francis Grondin Follow this and additional works at: https://digitalcommons.mtech.edu/grad_rsch Part of the Geological Engineering Commons MINERALOGY AND FLUID INCLUSION STUDY OF THE CRYSTAL MOUNTAIN FLUORITE MINE, RAVALLI COUNTY, MONTANA by Francis Grondin A thesis submitted in partial fulfillment of the requirements for the degree of Masters of Science in Geosciences: Geology Option Montana Tech 2019 ii Abstract The Crystal Mountain Fluorite Mine, known for its vast amounts of massively grown fluorite and rare accessory minerals including fergusonite (YNbO4) and thortveitite (Sc2Si2O7), is located in the Sapphire Mountains 22 km east of the town of Darby, Montana. This site has been mined previously between the years of 1954 through 1973. The Taber collection of drill core, thin sections, mine maps and miscellaneous reports on Crystal Mountain was recently donated to the Montana Bureau of Mines and Geology for future studies. The purpose of the present study has been to use modern methods of mineral identification and fluid inclusions on these samples to better understand the origin of the deposit. Hand samples collected from drill core and in the field were made into polished sections for analysis by SEM-EDS, Raman spectroscopy, and fluid inclusion microthermometry. The fluorite itself varies from colorless to deep purple, and is strongly enriched in yttrium (average of 1.51 wt% Y). The ore bodies were essentially pure fluorite in shallow-dipping masses up to 7 meters thick and > 40m in diameter. Impurities included silicate minerals (albite, phlogopite, quartz), Fe-oxides, titanite, apatite, thortveitite, sulfide minerals (pyrite, pyrrhotite, chalcopyrite), thorite, fergusonite, allanite, xenotime, monazite, and other REE-rich minerals. Thin sections of country rock show weak K- metasomatism in the form of orthoclase replacing plagioclase and phlogopite replacing amphibole. However, this alteration is subtle and is easily missed by visual inspection of outcrops and hand samples. Near contacts with country rock, fluorite is intergrown with silicate minerals in a texture that appears igneous. Two populations of fluid inclusions were found in the fluorite. Primary inclusions average 10-15m in size, are distributed randomly throughout the fluorite, and have homogenization temperatures from 350 to > 500ºC. All primary inclusions contain multiple daughter minerals including halite, sylvite, ankerite, siderite, and quartz. Salinities determined by the temperatures of halite dissolution ranged from 35 to 56 wt% NaCl. Secondary fluid inclusions, which occur along healed fractures in the fluorite, are bigger at an average of 30- 50m, with no daughter minerals. However, these inclusions contain a double bubble at room temperature which indicates a high CO2 content. Based on fluid inclusion geobarometry, the fluorite at Crystal Mountain formed at a minimum pressure of about 3 kbar, which corresponds to a depth of > 10 km. The above observations suggest that Crystal Mountain is a unique example of a magmatic fluorite deposit formed by crystallization of a fluoro-silicate melt that separated from a granitic melt. This model is consistent with the high temperatures, high pressures, and elevated concentrations of incompatible elements (Y, Nb, Ti, Sc, Th, REE) in the deposit, as well as the geometry of the massive fluorite ore bodies. Similar high temperature and high salinity fluid inclusions have been found in other fluorite deposits of western Montana, including the Snowbird, Spar, and Wilson Gulch deposits. More work is needed to determine the ages of these deposits and how they relate to one another. Keywords: Geochemistry, fluid inclusion geobarometry, magmatic fluids, fluorite melt, Idaho batholith, yttrium, scandium, rare earth elements iii Dedication This thesis could not be possible without the support of my loving mother Jeanne Prud’Homme and father Jean Grondin. I dedicate this solely to them since without their emotional and financial support I would not be here to complete a master’s degree in Geology and have a kickstart on my career. It is impossible to thank you enough for the morals and values you have taught me as a child. I could not have asked for better role-models. I also want to thank my brother Christian and friends, Stephen, Kristen, Christian, and Tyler. Their support has given me motivation and dedication to work hard. Thank you for all the laughs! iv Acknowledgements I would like to give a huge thank you to my advisor Dr. Chris Gammons for keeping me on track and making all of this possible. I would also like to thank my committee members Dr. Dick Berg and Dr. Larry Smith for their guidance during my thesis writing and Dr. Dick Berg for his great editing. A big thank you to Ms. Peggy Delaney for helping me find and organize all of the maps and cores I needed from the Montana Bureau and AMC collection. Dr. Gary Wyss was extremely helpful during SEM-EDS analysis with identification of minerals and instrument operation. Finally, thank you to the Stan & Joyce Lesar foundation for supporting and funding this research. v Table of Contents ABSTRACT ............................................................................................................................................. II DEDICATION ........................................................................................................................................ III ACKNOWLEDGEMENTS ........................................................................................................................ IV LIST OF TABLES .................................................................................................................................... VII LIST OF FIGURES ................................................................................................................................. VIII LIST OF EQUATIONS ............................................................................................................................. XI 1. INTRODUCTION ................................................................................................................................. 1 1.1. Thesis Statement ................................................................................................................ 1 1.2. History of the Crystal Mountain Mine ................................................................................ 1 1.3. Geologic Setting ................................................................................................................. 2 1.4. Previous Studies ................................................................................................................. 4 2. METHODS ........................................................................................................................................ 7 2.1. Historical Documents ......................................................................................................... 7 2.2. Sample Collection ............................................................................................................... 7 2.3. Optical Petrography ........................................................................................................... 8 2.4. Raman Spectroscopy .......................................................................................................... 9 2.5. SEM-EDS ............................................................................................................................. 9 2.6. X-ray Diffraction ............................................................................................................... 10 2.7. Fluid Inclusions ................................................................................................................. 10 2.8. S-isotopes ......................................................................................................................... 13 3. RESULTS ......................................................................................................................................... 14 3.1. Description of the fluorite bodies ..................................................................................... 14 3.2. Mineralogy ....................................................................................................................... 18 vi 3.2.1. Thin section petrography .................................................................................................................. 19 3.2.2. SEM results ........................................................................................................................................ 24 3.2.3. Raman Spectroscopy ......................................................................................................................... 29 3.3. Sulfur isotopes .................................................................................................................. 30 3.4. Fluid inclusions ................................................................................................................. 31 3.5. Other mineralogical work ................................................................................................ 35 3.5.1. Thortveitite .......................................................................................................................................